I am new to betta fish keeping, and my first betta lasted a week (swim bladder or strong filter). I got my second betta from Petco and I noticed the tail was rounded when it is supposed to be spiked, he is a crowntail. A few days later a small chunk seemed to be missing.

Housing
What size is your tank? 2.5
What temperature is your tank? ?
Does your tank have a filter? Not at the moment
Does your tank have an air stone or other type of aeration? No
Is your tank heated? No (House temp at 79)
What tank mates does your betta fish live with? None

Food
What type of food do you feed your betta fish? Hikari Gold
How often do you feed your betta fish? 6-8 a day, Fast on Wed.
Maintenance
How often do you perform a water change? Not enough time to tell
What percentage of the water do you change when you perform a water change? 30%-40%
What type of additives do you add to the water when you perform a water change? Aqueon Betta Bowl Plus

Water Parameters:
Have you tested your water? If so, what are the following parameters?

Ammonia: ?
Nitrite: ?
Nitrate: ?
pH: ?
Hardness: ?
Alkalinity: ?

Symptoms and Treatment
How has your betta fish's appearance changed? Not enough time to tell
How has your betta fish's behavior changed? Not enough time to tell
When did you start noticing the symptoms? Since I came home with him
Have you started treating your fish? If so, how? I don't Know how
Does your fish have any history of being ill? Not enough time to tell
How old is your fish (approximately)? It has been 5 days since I got him

Don't worry, just biting.. but there is probably a reason that involves his home so check if it's clean enough and that he's not bored :)
Add stress coat and keep his water clean and he'll be just fine :)

Oh yea if there's no filter do 100% or 90% if you don't want to take him out

Okay, just finished the water change so now he has nice, crystal clear water. I really hope the stress coat helps! I have to admit, the tank was pretty empty, but I got a nice big silk plant and a decorative fake wood thingy for him to hide and explore.

A question about the stress coat: How long do I keep adding it? Do I stop in a specific number of days or until his fins grow back?